This invention relates to computed tomographic (CT) imaging, and more particularly to methods and apparatus for the detection and diagnosis of head and neck abnormalities.
In spite of recent advancements in CT technology (faster scanning speed, larger coverage with multiple detector rows), energy resolution is still a missing piece, namely, wide x-ray photon energy spectrum from the x-ray source and the lack of energy resolution from CT detection systems. X-ray attenuation through a given object is not a constant. Rather the X-ray attenuation is strongly dependent on the x-ray photon energy. This physical phenomenon shows in the image as beam-hardening artifact, such as, for example, non-uniformity, shading and streaks. Some of the beam-hardening artifacts can be easily corrected, but some beam-hardening artifacts are more difficult to remove (i.e., correct). In general, known methods to deal with such problems include (1) water calibration, where each CT machine is carefully calibrated to remove beam hardening from materials similar to water, and (2) iterative bone correction, where bones are separated in the first-pass image, then beam hardening from bones are corrected in the second-pass. However, beam hardening from materials other than water and bone, such as metal and contrast agent, may be difficult to correct. Also, even with the above described correction methods, conventional CT does not provide quantitative image values, instead, the same material at different locations often shows different CT numbers.
Another drawback of conventional CT is a lack of material characterization. For example, a highly attenuating material with a low density can result in the same CT number in the image as a less attenuating material with a high density. Thus, there is little or no insight into what a material is made of based solely on the CT number. At least some state-of-the-art CT scanner currently available is limited to providing anatomical information. For head and neck scans, images produced by such scanners exhibit a significant level of image artifacts and CT number inaccuracy. These limitations prevent the utilization of the CT device for advanced diagnosis. Accordingly, the methods and apparatus described herein address the detection and diagnosis of head and neck abnormalities.